N‐Doped C@Zn₃B₂O₆ as a Low Cost and Environmentally Friendly Anode Material for Na‐Ion Batteries: High Performance and New Reaction Mechanism

Abstract Na‐ion batteries (NIBs) are ideal candidates for solving the problem of large‐scale energy storage, due to the worldwide sodium resource, but the efforts in exploring and synthesizing low‐cost and eco‐friendly anode materials with convenient technologies and low‐cost raw materials are still insufficient. Herein, with the assistance of a simple calcination method and common raw materials, the environmentally friendly and nontoxic N‐doped C@Zn₃B₂O₆ composite is directly synthesized and proved to be a potential anode material for NIBs. The composite demonstrates a high reversible charge capacity of 446.2 mAh g⁻¹ and a safe and suitable average voltage of 0.69 V, together with application potential in full cells (discharge capacity of 98.4 mAh g⁻¹ and long cycle performance of 300 cycles at 1000 mA g⁻¹). In addition, the sodium‐ion storage mechanism of N‐doped C@Zn₃B₂O₆ is subsequently studied through air‐insulated ex situ characterizations of X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), and Fourier‐transform infrared (FT‐IR) spectroscopy, and is found to be rather different from previous reports on borate anode materials for NIBs and lithium‐ion batteries. The reaction mechanism is deduced and proposed as: Zn₃B₂O₆ + 6Na⁺ + 6e⁻ ⇋ 3Zn + B₂O₃ ∙ 3Na₂O, which indicates that the generated boracic phase is electrochemically active and participates in the later discharge/charge progress.